This invention concerns improvements in or relating to floatable flexible hose and in particular to flexible hose of the type comprising a main fluid retaining carcass and a secondary fluid retaining carcass enclosing at least part of the length of the main carcass to prevent escape of fluid leaking from the main carcass.
Flexible hose of the type referred to above is particularly useful for transporting fluids the escape of which would constitute a severe health or safety hazard and is especially suited to applications where the hose is required to withstand a considerable amount of flexing in use, rough handling during connecting operations and severe weathering, for example in the hose line used to transport oil between a tanker moored offshore and an installation on shore. In assignee's earlier U K Patent Number 1 503 502 a flexible hose of the type referred to above is disclosed in which the secondary carcass is separate from the main carcass and constructed to expand radially under the pressure of fluid leaking through the main carcass which is retained by the secondary carcass. A hose of that type may optionally incorporate buoyancy material in the body of the main/or secondary carcasses. Alternatively or in addition buoyancy material may be provided between the main and secondary carcasses.
In a hose of the aforementioned construction the expansibility of the secondary carcass enables it to accommodate resiliently, and without damage by shock loads, the potentially sudden increase in pressure forces to which it may be subject upon a failure of the inner, main carcass.
That expansibility of the secondary carcass can be most undesirable for hose used underwater. As discussed in our GB Patent Number 2 117 4791 that expansion adversely affects the delicate balance which may be recovered between the buoyancy (displacement) forces and the weight of the hose if a hose line formed from a string of hoses is to maintain a desired configuration. To avoid or mitigate such problems it is taught in that patent that expansion of the secondary carcass relative to them a in carcass is constrained along the whole of the length of the hose.
In contrast to underwater twin carcass hose in which it can be desirable to avoid significant expansion of the secondary carcass, for floatable flexible hose of a kind adapted to float on the surface of water it is known to provide that the secondary carcass is relatively highly expansible. For example the secondary carcass typically may expand in diameter by 25% to 30% if exposed to the normal working pressure of fluid leaking through the main carcass.
That expansibility provides a potentially useful facility for detecting failure of the main carcass; that can be achieved by simple visual inspection without any need for mechanical or electrical leak detectors. However, although a 30% change in diameter of a secondary carcass is potentially well visible, if a thick layer of buoyancy material is provided around the secondary carcass that expansion may result in a change of only approximately 10% of the outer diameter of the hose, depending on the ratio of hose outer diameter to outer diameter of the secondary carcass. The percentage visible expansion is reduced still further if the buoyancy material is of a kind which compresses upon expansion of the secondary carcass. A consequence is that expansion of the floatable hose is not always clearly visible, especially when in rough waters.